I'm planning to build some ESL loudspeakers, and i'd like some input from the people here. There might be some flaws in my design with which you could help me.

ESLs have always interested me, and for long I've planned on building one. But I think that it's ugly to have a transformer between your amplifier and the ESL unit. So I designed an amplifier to drive them directly, with some high-voltage FETs (momentarily 10n100's, which shoud be able to handle an output of 800V, but I can easily put in transistors or FETs with higer voltages). At the moment I've ordered the first test PCB's, so now I've got time to build some good ESLs.

This low voltage-output means that I have to use a very small spacing between the stator and the membrane. Ideal would be to have a membrane-stator-distance of 0.5mm. (So I can use around 1kV on the membrane and 800Vpp audio)

At the moment I've already built a small unit with +/- 0.5mm spacing and a size of 20x40cm. This unit works, but leaks very much, it cant use more than 500V charge. But the sensitivity is quite good. At 400Vpp it is around loud enough for normal listening to speach or pop-music.

Now I plan to build a large 1mx1m full-range unit in the same way. My plan was the following:

Stator design
I planned to use something like this as a frame:
and on that I can glue a layer of 'woven wire' (how do i transloate the dutch 'gaas' to english?). I can buy sheets of woven wire, where the wires are around 1.7mm apart. It's still quite sturdy.
The spacing of the frame is around 3cm. So the woven wire is quite well supported. I hope...

spacers
I plan on using sheets of styrene as spacer. Just cut it up to 3mm broad strips running horizontally over the stator. I think it's easiest to glue them in position.
What kind of glue is good? At the moment I use bisonkit contact-glue. But this conducts a little bit, which is bad.

membrane
I've bought some mylar from the flowershop. Its around 10g/mm^2. Is this too heavy?
I've calculated that I need around 2kg of tension no the membrane in the vertical direction. I don't yet really know what's the best way to get tension on the membrane. But hanging it down with a weight, and then glueing it to the stator seems best to me...
As coating I'll use kontaktchemie 100 antistatik.

I've done some calculations on ESLs, and i get the following (theoretical) specifications.
max. sound pressure: 104dB (in practice more likely is 100dB)
Capacitance: 8.8nF
lowest frequency due to limited movement of membrane: 55Hz (more likely to be 70 Hz)

The panel won't output frequencies lower then +/-70Hz in this configuration. (I could make that 35Hz If I double the spacer thickness, but that costs me 6dB sound pressure)
What's the relation between the size of the element and the lowest frequency due to phase-cancellation? I wasn't able to include that in my simple equation.

One couldnīt give serious advice with just the few details you mentioned.
I think, that the way You start the design might not be successful, because You donīt start at the beginning of a typical design process but in the midst.
Choosing some material and hack them into something together will not result in a good ESL design, but just a īsomehowī working prototype.
Since You already built a prototype and from the way you asked, I assume You want to build a proper design this time? But what? I strongly recommend to not start Your ESL-life with the try to build a big fullrange ESL from scratch. The end result will most definitely not be optimal.
Think of hybrid panels first. There are a lot of Pros for hybrid panels and a lot of Cons to fullrange panels.

First think about a spec-sheet which should contain the aims You want to reach in detail, for example which frequency range needs to be covered, size constraints, drive constraints etc. etc.
Then think about rough design outlines, like the dimensions, which kind of stator fits the demands, cost, build effort, sourceabilty, etc. etc. There are a lot of tips in this forum about design guidelines and material-choices. Invest some time and the search-function is Your friend
Then Iīd start looking for materials that might suit the demands. Mylar from a Flowershop wonīt be suitable material......if it is Mylar anyhow.
Again there are a lot of tips in this forum about sources where to get and to buy quality material from.
The better Your work in these pre-building stages is the higher will be the probability of success. The actual building process then becomes just a minor problem.

At the moment I'm already listening to a hybrid system which I bought and repaired for very little money. (final 80i + cheap, bad subwoofer)

I want to go bigger now. So I'd like to build a full-range unit, but I want to build it suitable for my direct-drive amplifier. This gives the limitation of 800Vrms audio. (comparable with a 50W amp with a 1:40 step-up)
As far as I've read this would be enough for a full-range system.
I know this is quite difficult. I don't mind it not being perfect the first time. I just don't feel like building something I already have (a hybrid design)

Of course I can make a spec-sheet, and all that paperwork. But my experience (in electronics) learned me that doing that is only useful when you exactly know where the problems are going to be. Which is only the case if you've built some ESLs already.

Most difficult is the low-frequency-repsonse. But I can't really find much information about that. I could calculate/simulate it myself, but that would take quite a lot of time.. And there is a large chance of mistakes, so i wouldn't even trust my calculations...

A problem is that quite a lot of the theory on the internet (in my humble opinion) is not correct. So I don't really know who to trust.
For practical building tips, there's of course a lot of information. But most involve a lot(!) of work. The design I propose here is a lot less work. I might be able to completely build a unit in 1 day this way

I've already found out that I really should use a better diaphragm, and a better coating. (which didn't surprise me, calculations tell the same )

My main question is: Is it quite possible to build an ESL with only 0.5mm-0.7mm clearance between stator and diaphragm? (I don't think many people have tried that before.)

The "woven wire" you mentioned sounds like it will have some sections of wire closer to the diaphragm than other sections. If that's true, you are likely to run into some limitations that otherwise wouldn't trouble you. The points where the woven wires are close to the diaphragm will limit the diaphragm's displacement and the magnitude of the bias voltage you can apply, and the fact that other regions of wire are farther away from the diaphragm means that you'll sacrifice some of the panel's sensitivity--especially if you are going to use very small diaphragm-stator spacing. If the variation in wire-to-diaphragm distance were small compared to the average wire-to-diaphragm distance, it might be less problematic. In your application, it seems to me it will likely cause difficulties. On the other hand, if you can build one in a day, it won't cost much time to try it out.

- simple dipole: thatīs the ESLīs natural way to work, go ahead.
- 70Hz-20kHz: quite sensible choice, which allows to stay within reasonable dimensions. Rough suggestion: 0.3-0.5mē and d/s: 2-2.5mm
- 800V (I assume peak-peak, and how much current capability?) would only be sufficient to drive a small tweeter or headphone sized panel with very small d/s distance of <1mm to the suggested SPL-levels. For a d/s of 2-2.5mm rather calculate with 4000Vp-p. I wouldnīt drive a panel with a d/s of 0.5-0.7mm d/s lower than 400-500Hz.
The lower frequency limit is no matter of the d/s. The d/s only defines the dynamic range, i.e. the maximum SPL-level that can be reached at a certain frequency. Excursion demands increase with lowering frequency.
- cheap: Besides material You might get for free, a wire stator design is one of the cheapest possibilities that allows for good end results.
- woven wire or a woven mesh was tested against other stator-designs by Matthew Lattis in audioXpress. Efficiency was stated below par. The uneven surface and as such varying distances of d/s could be a reason for that. As a drawback I regard that itīs nearly impossible to insulate such a mesh structure. On the other hand could a very fine mesh not only supply for a very homogenous electrical field (needed the more, the smaller the d/s) but also for a mechanical damping.
- 0.5-0.7mm d/s is typically used with small tweeter- and headphone Panels, but hardly ever with larger panels. At one will a very high degree of mechanical precision be needed (increased costs and effort), at second will such a small d/s seriously restrict the bandwidth towards lower frequencies and at third will such a panel show a high capacitance value and as such represent a very difficult load to drive. The current needed to drive a capacitance depends on the capacitance value, the drive voltage, the frequency and the signal shape. For sinusiodial signals the formula reads: Ipeak= C*pi*f*Vpp
Example: symmetrical ESL-panel, panel diaphragm size: 0.5mē, d/s: 0.7mm -> C~6.3nF, Vpp: 800Vp-p, f: 20kHz.
Ipeak= 6.3e-9*3.14*2e4*8e2 = 1.6e-1Apeak = 160mApeak.
If You fudge a bit and define 20kHz as the -3dB frequency (definition of bandwidth limit) the current demand halves to 80mApeak. Still though this is a current value most ESL-amps canīt supply for.

I have not made any ESLs yet, just repaired my Quad ESL. So I am a beginner too. I recommend you to invest in a micrometer screw thickness gauge. Fold the mylar 8x or 16x so you can measure precisely. Second, mylar from the flower shop is simply not good. You need 10-12 um thickness for the bass section and 2.5 to 5 um for the treble. Calvin, correct me if I am wrong. Also you can try Johnson's Pronto Antistatic spray for diaphragm treatment. Similar to your stator structure was available over here as part of fluorescent lamp armatures (light diffusor grid) and I was also thinking about experimenting with them in an ESL. Is the more recent Quad ESL63 using something similar?

The amplifier is at this moment capable of ~200Vrms, with ~300V supply voltage. (600Vpp) This is because it's a bridged amplifier, otherwise you'd never get the balanced input-signals an ESL requires... (wouldn't be a real problem in constant charge mode, but who cares; more decibels )
When I've got the PCB's I dare go to higher voltages, and if I change the transistors I should be able to supply 2.4kVpp -> ~800Vrms output.

The maximum current is around 200mA. Which wouldn't be enough according to your calculations, because the amplifier is in bridge mode.
But the max current is (for music) just not needed. A normal stepup transformer couldn't supply those currents either, because of wire resistance. I don't bother getting it higher, although with small modifications >1A would be easily possible with this design. But going to these values is dangerous for the transistors. A strong 20kHz note would fry the transistors with around 1kW of peak dissipation. If needed I can upgrade the current capability, but this is not needed. So let's say 'current capability is sufficient'.
I won't make 20kHz a -3dB-point. I'd rather let my amplifier have its -3dB@40kHz but just let it go into a current-clip at very high frequencies. This gives the best sound quality (as long as you don't have more than 400Vpp of 20kHz, but what hellish music would sound like that ) Because there is no transformer means things are quite different.

And now for some theory about the lowest frequency. Excursion demands are indeed the most important thing here.

- At some frequency the excursion becomes so large that tension force becomes larger than the electrical force. When both forces are equal you're at the -3dB (or -6dB?) point. (here the force to make a sound pressure wave is included in the excursion)
- The tension can be taken lower at larger d/s, this is the reason that at low d/s lower frequencies can be used. And the tension should be taken as low as possible! This makes a difference in distorsion.
- I see a lot about resonance-frequencies, these are unimportant if frequencies well below resonance are still at smal excursions. The resonance is very much overdamped.
- One BIG problem is that the in a lot of dipole designs air can go from the front to the back of the panel without a problem. This means far less weight of air being moved -> higher excursion. But this effect I'm not able to calculate easily, so I ignore it. In my calculations I still make the assumption that the panel is much bigger than wavelength.

You're right about the woven wire, it is actually not that good at 0.5mm. It would probably be optimal at 0.8mm. (well, larger is of course always possible)
It's of good quality very regularly woven, it can easily be made totally flat if glued onto something. I think less then 0.2mm accuracy can easily be achieved with this material.
It is very much like this:

[QUOTE=zweetvoetje; I think less then 0.2mm accuracy can easily be achieved with this material.

Hello Zweetvoetje,

I have made my ESL's with metal grids like you propose, but with insulated wire glued on them (and segmentation). They were presented at the last ESL-club meeting in june in Den Haag. (I must try to make some new photo's)
In my case these grids are 22.5x177.5cm with a mesh grid of 44.4mm.

My experience tells me that your estimated accuracy is by a large amount overestimated. Even if your fine mesh could be very accurate, the metal grids are industrial products with rather large tolerances and hardly flat (at least the ones I have, but on construction sites I did not yet find others with tighter tolerances. Besides that the metal grids are hardly cheap, unless you can get them as overstock or something.

another problem of the wire mesh could be that it does not remain flat after gluing it to the supports. It might need to be stretched before gluing, but that could prove to be problematic too.

If you really want to go down this route (and I can't blame you, it's certainly a fast way to build a stator), you could consider using plastic mosquito mesh (used to install in window frames used to let fresh air into the room). Those would be easy to stretch a little (so they don't bulge after gluing), and are probably flatter than wire mesh. They could be sprayed with conductive paint after gluing.

Oh and as others said, mylar from the flower shop is WAY too thick, you really need the thin tensilized mylar stuff (6 or 12 micron for a full range ESL would be okay).

Best,
Kenneth

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